This invention relates to electrical tissue stimulators and more particularly, to tissue stimulator output circuits and sense amplifier circuits.
Electrical body tissue stimulators, such as nerve or muscle stimulators, cardiac pacemakers and the like are well known in the prior art. Electrical energy has been applied to body tissue using batteries, condensers, electrostatic charge generators and alternating current generators either for scientific curiosity or for treatment of various injuries and illnesses. Condenser discharge impulses of electrical energy into body tissue have been used therapeutically since the invention of the Leyden jar and their use continues today in implantable body stimulators, particularly cardiac pacemakers.
Early implantable cardiac pacemakers, such as those shown in U.S. Pat. No. 3,057,356 and subsequent pacemakers up to the present date comprise small, completely implantable, transistorized and battery operated pulse generators connected to flexible leads bearing electrodes directly in contact with cardiac tissue. Demand cardiac pacemakers have traditionally employed a timing circuit, a stimulating circuit and a separate sensing circuit, all of which draw current from the power source. The stimulating circuits of such pulse generators have traditionally comprised constant current or constant voltage output circuits employing output capacitors which are charged to battery potential through a relatively high impedance and discharged through electrodes in contact with myocardial tissue to stimulate depolarization of the tissue. The output capacitors are typically recharged during the intervals between successive discharges.
The discharge of an output capacitor through the myocardial tissue results in after-effects due to the disruption of the electrical equilibrium condition at the tissue-electrode interface and the polarization of the tissue's intrinsic dipole moments. Post relaxation of these stimulation based after-effects, traditionally characterized as "polarization", manifest themselves to traditional pacemaker sense amplifiers coupled to the stimulation electrode as voltage signals which persist for a period of time following delivery of stimulation pulses. In traditional pacemakers, these after-effects interfere with the pacemaker's ability to sense depolarizations of the heart during, closely following or caused by delivery of stimulation pulses.
Various attempts were made in the prior art to counteract the "polarization" after-effects of the stimulation pulse and simultaneously recharge the output capacitor by means of a fast recharge pulse delivered through the stimulation electrodes following the trailing edge of the output pulse, as exemplified by U.S. Pat. Nos. 4,476,868, 4,406,286, 3,835,865 and 4,170,999. However, simply passing sufficient current through the electrode-tissue interface to recharge the output capacitor does not necessarily return the electrodetissue system to its prior electrical equilibrium condition. Alternatively, it has been suggested to counteract the after-effects of delivery of a stimulation pulse by simply tying the electrodes involved in delivery of the pulse together following delivery of the pulse, as disclosed in U.S. Pat. No. 4,498,478 issued to Bourgeois or by means of a train of low energy pulses as disclosed in U.S. Pat. No. 4,811,738, issued to Economides, et al.